This invention relates to the hydrogenation of phenol and, more particularly, to the control of the hydrogenation of phenol to cyclohexanone in the presence of a promoted palladium catalyst.
In the hydrogenation of phenol employing a palladium catalyst, the activity of the catalyst, and hence the rate of hydrogenation, decreases with continued use of the catalyst due to impurities present in the hydrogenation reaction mixture which poison the catalyst. While processes, such as those disclosed in U.S. Pat. Nos. 3,692,845 and 3,187,050, have been developed to purify organic compounds such as phenol to be hydrogenated, the poisoning of metallic catalysts has not been entirely eliminated in large scale commercial processes due to long-term accumulation of impurities, such as those impurities which are introduced with the phenol and the hydrogen gas, and those impurities which are produced during the processing.
To avoid the economically prohibitive alternatives of discarding poisoned catalyst or continuing to use the poisoned catalyst at a reduced rate of hydrogenation, it is desirable to promote the rate of hydrogenation, thereby overcoming the disadvantages of continued use of such poisoned palladium catalysts. The hydrogenation of phenol to cyclohexanone has been promoted by the use of "promoted palladium-on-carbon catalysts", i.e., catalysts which have been treated prior to their addition to the hydrogenation reaction mixture, to incorporate on the catalysts a material which enhances their activity. Thus, in U.S. Pat. No. 3,076,810, cyclohexanone is produced by hydrogenating phenol using a sodium-promoted catalyst, i.e., a palladium catalyst which has been modified prior to its introduction to the reactive mixture, to incorporate sodium thereon. Alkaline reacting agents in limited amounts are also disclosed as being added to assist in promotion when the sodium-promoted catalysts of that reference are employed. However, such catalyst systems have not been entirely satisfactory, and research has continued to develop an improved process and/or catalyst.
Following the Flixborough, England disaster in 1974, which resulted in loss of lives and equipment destruction, the inherent danger involved in synthesis of cyclohexanone using high temperature, liquid phase processes became clearly evident, i.e., the potential formation and detonation of organic vapor clouds was fully recognized and defined:
(a) Leakage of process vapor has proved to be a problem that may be solved within the constraints of existing technology; it requires vapor detection devices and combative actions, such as automated unit isolation, system shutdown, or water fog vapor suppression.
(b) Cataclysmic rupture of liquid lines and vessels containing volumes of organic liquids above the atmospheric boiling point has proved to be a very serious problem within the constraints of existing technology. Intrinsic safety requires operating temperatures to be at or below atmospheric boiling point of the reaction mixture in each vessel. However, in known processes, such lower operating temperatures greatly reduce production capacity.
U.S. application Ser. No. 886,718 filed Mar. 15, 1978, relates to a highly active catalyst for selective hydrogenation of phenol to cyclohexanone. The catalyst comprises 0.2 to 10 weight percent of palladium, based on the total weight of the catalyst, supported on carbon particles having diameters of 3 to 300 microns and a surface area of 100 to 2000 m.sup.2 /gram, said catalyst being promoted by sodium in an amount of at least 1000 ppm. Preferably, said sodium-promoted palladium catalyst is additionally promoted during said hydrogenation by contacting the catalyst with phenol containing a small amount of an in situ promoter selected from the group consisting of sodium hydroxide, sodium carbonate, and sodium phenate, said amount being 10 to 300 ppm in terms of sodium of said in situ promoter.
The highly active catalyst of U.S. application Ser. No. 886,718 is an important contribution to this art because it permits hydrogenation of phenol with reduced amounts of catalyst and with intrinsic safety by operating at temperatures at or below the atmospheric boiling point of the reaction mass. However, we have found that control of the hydrogenation reaction is difficult with use of the highly active catalyst, and research has been continued to develop an improved method for controlling the process.